Delving into climate dynamics: Isotope analysis and observations unravel paths and drivers of Singapore rainwater

20 Jun 2024

Authors: ZHANG Yi Lin and Bernie WEE Wei Ken

Contributor: Lauriane CHARDOT

The rain that lands in Singapore has undergone a long journey. For example, during the Northeast Monsoon from December to March, water mainly evaporates from the South China Sea, gets transported and mixed in the atmosphere. As the vapour then cools at high elevation, it condenses into water droplets before falling in Singapore. While we know the general patterns, we do not fully understand the intricacies of the atmospheric processes over Southeast Asia. This has hindered enhancements of climate models for predicting future climate change, improving weather forecasts, and understanding past climate records. 

We recently found that rainwater collected in Singapore contains clues needed to unravel complex regional atmospheric processes. In a study published in Journal of Geophysical Research - Atmospheres, we explained what led to six cold surge rain events during the Northeast Monsoon in 2017 and 2022 in Singapore. 

Monsoon surge rain event in Singapore (Source: Dewang Gupta/Unsplash).  

But how do we extract information about what goes on in the atmosphere from rainwater? We look closely at its composition. While we likely learned that the chemical formula for water is H2O, a few forms of H2O exist due to the diverse distribution of variants of both hydrogen and oxygen, the two elements that make up water. These variants can exist in slightly different atomic masses called isotopes. These isotopes mix in various ways due to a range of atmospheric processes, such as evaporation and condensation, creating different versions of the water molecule. By analysing the ratios of these isotopes in rainwater, we can deduce the relationship between them and the climatic data, ultimately leading to a better understanding of the processes in the atmosphere.  

The first three authors of the study, Ms Yilin Zhang, Dr Shaoneng He and Mr Bernie Wee (from right to left), analysing samples in the lab (Source: Yunyue Yang/NTU Singapore) 

In our study, we collected rainwater at Nanyang Technological University Singapore during six cold surge‐related rain events on 23rd and 25th of January 2017, 2nd, 3rd and 7th of March and 20th December 2022. The samples were collected every 3 to 5 minutes, to understand how isotope ratios changed over time from the start to the end of the rain events. We also compared the samples with each other to assess any trends or similarities. 

(Left) Map showing the location of the sampling site at NTU, Singapore (from He et al., 2021). (Right) Photo of the rain collection panel designed for continuous and high-temporal resolution sample collection, located outside the Asian School of the Environment building at NTU Singapore (Source: Yilin Zhang/NTU Singapore)  

For each rainwater sample, we analysed the oxygen and hydrogen isotope ratios, and how these ratios vary with time for each event. Using satellite images of the precipitation and back-trajectory data of air masses, we reconstructed the path the water had taken. From this reconstructed pathway, we examined the processes occurring along the trajectory and at the precipitation site that affected the rainwater isotopes. By better understanding these processes, we can improve the accuracy of some parameters used in climate models.  

Map showing the deduced tracks of the rainwater from the analysis, that reached Singapore (red triangle) (Source: Yilin Zhang/NTU Singapore)   

By looking even more closely at the chemistry of the rainwater, we found that complicated hydrological processes can alter the isotope ratios of rain in the tropics. For example, if rain evaporates while it is falling at a location, certain isotopes of the rainwater return to the rain clouds and the next rainfall might have more of those isotopes. In our samples, we found that δ18O, an isotope ratio of oxygen, increases in rainwater when rain evaporation is significant while d-excess, a secondary isotope parameter, decreases. This relationship between δ18O and d‐excess indicates that rain evaporation plays a crucial role in controlling precipitation isotopes in Singapore.  

We also found that processes that happen in clouds, such as the evaporation of raindrops and the recycling of water vapour during convection patterns, can raise the levels of 17O, another isotope of oxygen, of the downstream vapour and subsequent rain. 

We are now working on incorporating 17O in global climate models and expect to shed more light on the complex relationship between 17O and tropical convection. 


Illustration of how rain evaporation can affect the isotope composition of subsequent rainfall events (Source: Yilin Zhang/NTU Singapore) 

Our work shows that the amount of 17O in Singapore rainwater is strongly influenced by convection systems occurring upwind from Singapore. This insight is crucial for understanding the origins of rain in Singapore and should be considered in future analyses of local rain patterns.  

Our findings also provide valuable tools for comprehending the complex atmospheric processes that drive rain in the region. They enhance our overall understanding of tropical climate dynamics and we hope they can contribute to better interpret and forecast weather and climate changes in Southeast Asia. 

This work was supported by a Singapore MOE Tier 2 Grant (MOEMOET2EP10121‐0008), a Singapore MOE Tier 3 Grant (MOE2019‐T3‐1‐004) and a Stephen Riady Geoscience Scholarship. 

Blog Category


Geographic Area

Asia > Southeast Asia



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